Pollutants are carried from the ocean through mountain passes on an almost
daily basis during the summer months.
Streams of air carrying Bay Area emissions mix with locally
generated pollution from automobile traffic, small engine exhaust, industry,
and agriculture in the Valley and are diverted both north and south.
A warm inversion layer acts like a blanket on the smog layer,
preventing it from dissipating higher in the atmosphere. Because of high
pressure, the Central Valley regularly experiences these thermal inversions.
The Valley, which is nearly at sea level, often fills at night with cool
heavy air underneath a layer of warmer air. The cool air layer grows through
the night reaching up to 3000 feet thick.
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